How to quickly jump from one entry box to another?
The eyedropper button next to each setting activates an operator, which means you can connect it to a keyboard shortcut.

In the preferences under Input you can add a new keyboard shortcut. I would suggest adding it under 3DView/ObjectMode. By scrolling down to the bottom you will find an Add New button which you click and then you fill in the details.

For the operator name, which starts as none, enter cgcookie.eye_dropper, then the target_property will become available where you enter source_object to set the source and target_object for the target. Then choose the shortcut settings you want to use for each. By clicking the Save User Settings button, the shortcut will be available every time you start blender.

Here I set &#9095; AltW to set the source object and &#9095; AltE to select the target object.



Now you can press &#9095; AltW and click an object to set it as the source object and &#9095; AltE to choose the target object.